1. Field of the Invention
The present invention relates to a liquid crystal display panel utilizable as a touch panel or for the display of televised images, a projection type display device utilizing the liquid crystal display panel as a light valve for the display of images in the form as projected onto a screen, and a viewfinder utilizing the liquid crystal display panel as a display monitor for monitoring images being videoed.
2. Description of the Prior Art
The liquid crystal display panel is known to be light-weight and thin in thickness as compared with a cathode ray tube and a variety of liquid crystal display panels have been developed. Recent application of the liquid crystal panel is a display unit in pocketable television receiver sets and a display unit in viewfinders of video cameras. However, the liquid crystal display panel has difficulty in securing a relatively large display format and, therefore, a compromise has been made to incorporate the liquid crystal panel in a projection type display device of a type employing an optical system for projecting images generated by the liquid crystal panel onto a screen. The projection type display device and the viewfinder both currently placed in the market make use of a twisted nematic (TN) liquid crystal display panel which utilizes a rotatory polarization of the liquid crystal.
The prior art liquid crystal display panel will be discussed in detail with reference to the accompanying drawings which illustrate it only for illustration purpose.
FIG. 60 illustrates a fragmentary sectional view of the prior art liquid crystal display panel. The prior art liquid crystal display panel makes use of a layer of TN liquid crystal 383 filled and sealed between an array substrate 12, formed with thin-film transistors 14 and others, and a counter substrate 11 spaced a distance of 4 to 6 .mu.m from the array substrate 12. A peripheral region of the TN liquid crystal display panel outside a display region is sealed by the use of sealing resin (not shown). Reference numeral 381 represents a black matrix formed of metallic material such as chromium; reference numeral 13 represents a counter electrode made of transparent material such as ITO; reference numeral 15 represents pixel electrodes; and reference numerals 882a and 382b represent orientation films.
The orientation films 382a and 382b are formed so as to overlay the pixel electrodes 15 and the counter electrode 13 and are subjected to a rubbing treatment to orient liquid crystal molecules of the liquid crystal layer 383. A polarizing plates 384a and 384b are lined to each of the counter substrate 11 and the array substrate 12 respectively.
The prior art TN liquid crystal display panel discussed above is manufactured in the following manner. Each of the array substrate 12 and the counter substrate 11 is lined with an orientation film 382a and 382b which is subsequently rubbed for alignment treatment. Then, a peripheral region of the array substrate 12 is deposited with a sealing resin (not shown) leaving an injection port for subsequent injection of a quantity of TN liquid crystal material 383. On the other hand, beads of transparent glass or synthetic resin are distributed over the counter substrate 11 so that a uniform thickness of the TN liquid crystal layer can be formed. Thereafter, the counter substrate 11 and the array substrate 12 are joined together, followed by heating of the sealing resin to cure the latter to thereby firmly bond the substrates 11 and 12 together. The assembly of the substrates 11 and 12 bonded together is placed in a vacuum chamber to evacuate the space between the array substrate 12 and the counter substrate 11, followed by immersion of the assembly into a bath of TN liquid crystal material. When the vacuum in the chamber is released, the TN liquid crystal material is sucked into the space between the substrates 11 and 12 through the injection port, followed by sealing of the injection port to thereby complete the liquid crystal display panel.
FIG. 15A illustrates a plan view of a display panel module in which the liquid crystal display panel is fixedly mounted on a chassis and FIG. 15B illustrates a cross-sectional view taken along the line 15B--15B in FIG. 15A. A chassis 161 in the form of a stainless metallic plate has a printed circuit board 162 mounted thereon. The printed circuit board 162 has a connector 163, electrolytic capacitors and others mounted thereon and also has a number of copper conductors (not shown) formed thereon by the use of a circuit printing technique for transmission of electric signals from the connector 163. The printed circuit board 162 has a central region perforated, and the TN liquid crystal display panel is mounted on the printed circuit board 162 with the display region thereof aligned with the central perforation defined in the printed circuit board 162. The TN liquid crystal display panel has thin-film conductors formed therein for transmission of the electric signals. The copper conductors are connected at one end with the associated thin-film conductors by means of fine wires (bonding wires) made of aluminum.
The peripheral region of the TN liquid crystal display panel outside the display region thereof is formed with a generally square ring-shaped light shielding pattern 164 which is indicated by dotted lines in FIG. 15A. Cross-sectional representations of the light shielding pattern 164 taken along the lines corresponding to lines 16A--16A and 16B--16B in FIG. 15A are shown in FIGS. 61A and 61B, respectively. The light shielding pattern 164 made of chromium and having a film thickness of about 1,000 angstrom is formed over the counter electrode 13 on the counter substrate 11. Reference numeral 21 represents source signal lines.
The reason for formation of the light shielding pattern 164 and the black matrix with the use of chromium is because a relatively small film thickness can be obtained with an increased light shielding effect. The TN liquid crystal display panel requires an orientation treatment to be effected to the orientation films 382 to align the liquid crystal molecules. The orientation treatment is carried out by rubbing the pixel electrodes 15. If the black matrix 381 has a relatively great film thickness, it gives rise to considerable surface irregularities on each of the substrates 11 and 12 and a favorable orientation treatment can no longer be effected.
The reason for the formation of the light shielding pattern 164 in the peripheral region of the TN liquid crystal display panel outside the display region will now be described briefly. The peripheral region of the TN liquid crystal display panel outside the display region has no pixel, but is formed with the source signal lines 21 for transmission of electric signals to the display region. Since the source signal lines 21 are in the form of a metallic thin film, they shield light off. However, light incident upon a gap between one source signal line and the neighboring source signal line is allowed to pass therethrough since no light shielding element exist in that gap. Passage of light through the display region of the TN liquid crystal display panel suffices, and light transmitted from somewhere other than the display region of the TN liquid crystal display panel is of no use and may constitute a cause of darkening of images being displayed. The light transmitted from somewhere other than the display region of the TN liquid crystal display panel is shielded by the chassis 161. Since the chassis 161 is in the form of a stainless metallic plate, no light passes therethrough. The chassis 161 is perforated at a central area thereof which is positioned so as to align with the perforation in the printed circuit board and the display region of the TN liquid crystal display panel.
Although the TN liquid crystal display panel has dimensions highly precisely tailored since it is manufactured by the use of a semiconductor process, the accuracy to which the chassis 161 is formed is low since the chassis 161 is manufactured by the use of a mechanical process. Also, the accuracy to which the TN liquid crystal display panel is fitted to the chassis 161 is low. Accordingly, if the perforation in the chassis 161 is great in size, the light leaks from the peripheral region of the display panel outside the display region, that is, gaps between the neighboring source signal lines. On the other hand, if the perforation in the chassis 161 is small in size, the chassis 161 will shield a portion of the display region of the TN liquid crystal display panel. Also, any error in fitting the TN liquid crystal display panel to the chassis through the printed circuit board may result in leakage of light from one end while the opposite end will shield a portion of the display region of the display panel. Therefore, while the light shielding pattern 164 is formed to have a width of about 2 mm, the perforation in the chassis 161 is so chosen to have a size greater than the display region of the display panel. This practice makes it possible to allow the light shielding pattern 164 to occupy a position between the perimeter of the perforation in the chassis 161 and that of the display region of the display panel and neither leakage of light from the outside of the display region nor shielding of that portion of the display region of the display panel will likely occur even though the display panel is fitted displaced 2 mm at most from the design position.
The display panel utilizing the TN liquid crystal material requires the use of a polarizing plate 384a to convert the incident light into a linearly polarized light. Also, another polarizing plate 384b is also required on an exit side of the liquid crystal display panel to detect light modulated by the liquid crystal display panel. In other words, the TN liquid crystal display panel requires the use of the polarizing plate 384a (hereinafter referred to as an polarizer) for linearly polarizing the incident light and the polarizing plate 384b (hereinafter referred to as an analyzer) for detecting the modulated light to be disposed on incident and exit sides of the TN liquid crystal display panel. Assuming that the pixel opening of the liquid crystal display panel is 100% and the amount of light incident upon the polarizer 384a is given 100, the amount of light emerging from the polarizer 384a is 40%, the transmittance of the display panel is 80% and the transmittance of the analyzer 384b is 80%, and therefore, the transmittance as a whole is about 25%, that is, (0.4.times.0.8.times.0.8.apprxeq.0.25). This means that only about 25% of the total light incident upon the TN liquid crystal display panel is utilized effectively and, accordingly, the TN liquid crystal display panel has a problem in that images are apt to be displayed at a low luminance.
Light lost as it passes through the polarizing plate 384 is substantially absorbed by the polarizing plate 384 and converted into heat which in turn heats the polarizing plate 384 and the display panel itself by radiation. In the case of the projection type display device, the amount of light incident upon the polarizing plate 384 amounts to some ten thousand luxes. Accordingly, where the TN liquid crystal display panel is used as a light valve in the projection type display device, the polarizing plate 384, the panel and others are heated to a temperature sufficient to cause them to deteriorate in a relatively short period of time.
Also, the TN liquid crystal display panel requires lining of the orientation film 382 which must be rubbed subsequently. The use of the rubbing process results in increase of the number of manufacturing steps which in turn brings about increase in manufacturing cost. On the other hand, it is a recent trend that the number of the pixels in the display panel used in the projection type display device amounts to 300,000 or more and the size of each pixel is correspondingly reduced. Reduction in pixel size in turn brings about an increased number of surface irregularities per unit area resulting from signal lines, thin-film transistors and other elements. The presence of the increased surface irregularities obviously hampers a satisfactory rubbing. Also, reduction in pixel size brings about reduction in pixel opening since the area of formation of the thin-film transistor 14 and the signal lines for each pixel increases. By way of example, in the case where the display panel of 3 inches in diagonal size is formed with 350,000 pixels, the pixel opening is about 30%, and where the same display panel is formed with 1,500,000 pixels, it is indicated that the pixel opening will be about 10%. Reduction in pixel opening brings about not only reduction in luminance of the images being displayed, but also accelerated reduction in performance of the TN liquid crystal display panel as a result of heating of the display panel under the influence of light impinging upon areas other than the area of incidence of light.
The TN liquid crystal material modulates light with change in orientation of liquid crystal molecules that takes place when a voltage is applied to the pixel electrodes 15. As indicated previously, the TN liquid crystal display panel makes use of the polarizing plates disposed on the incident and exit sides thereof, the axis of polarization of the polarizer 384a lying perpendicular to that of the analyzer 384b. In general, the TN liquid crystal display panel is used in a mode (NW mode) in which a black display can be effected upon application of a voltage. While the display panel useable in the NW mode is considered excellent in respect of color reproducibility of images being displayed, it has a problem in that light tends to leak from around each pixel. This is because the liquid crystal molecules do not align in a normal direction, but in a reverse direction. This alignment is referred to as a reverse tilted domain which occurs when the direction of set up of the liquid crystal molecules is partially reversed under the influence of an electric field developed between the pixel electrodes 15 and the signal lines 21. Portions of the liquid crystal molecules which set up in the reverse direction allow the light to pass through the analyzer 384b on the exit side of the display panel even though an electrical voltage is applied thereto. In other words, light leakage occurs, which does not occur if the liquid molecules are set up in the normal direction.
One method of avoiding the light leakage is to increase the width of the black matrix 381 that is formed over the counter electrode. However, this is not an effective method since the increased width of the black matrix 381 results in reduction in the area of closure of the pixels which in turn brings about reduction in luminance of the images being displayed.
The display panel utilizing the TN liquid crystal material as hereinafter described requires the use of the polarizing plate 384. Also, since the light leakage tends to occur around the pixels, the black matrix must have an increased width. Accordingly, the efficiency of utilization of light is low and the display luminance is low. Moreover, light impinging upon the black matrix does in turn heat the display panel to an elevated temperature which subsequently brings about reduction in lifetime of the display panel as a whole.
Similarly, the projection type display device in which the TN liquid crystal display panel is used as a light valve has a low efficiency of utilization of light, accompanied by reduction in luminance of the images being projected onto the screen. In view of this, the projection type display device utilizing a polymer dispersed (PD) liquid crystal panel that requires no polarizing plate 384 has been suggested and disclosed in, for example, the Japanese Laid-open Patent Publication No. 3-94225. The PD liquid crystal display panel used as a light valve in the projection type display device modulates the incoming light by scattering or transmitting the incident light.
The PD liquid crystal display panel is of a structure which is similar to the TN liquid crystal display panel shown in FIG. 60, but differs therefrom in that the polarizing plate 384 and the orientation film 382, both shown in FIG. 60, are dispensed with. As a matter of course, the PD liquid crystal display panel makes use of polymer dispersed liquid crystal material.
Operation of the PD liquid crystal display panel will be described briefly with reference to FIGS. 34A and 34B which illustrate explanatory diagrams. As shown therein, a quantity of polymer 332 has liquid crystal droplets 331 dispersed therein. Pixel electrodes 15 are connected with thin-film transistors (not shown) which, when they are switched on and off, apply a voltage to the associated pixel electrodes 15 to vary the direction of orientation of the liquid crystal aligned with the pixel electrodes 15 to thereby modulate the incoming light. So long as no voltage is applied as shown in FIG. 34A, the liquid crystal droplets 331 are randomly oriented in varying directions. In this condition, a difference is created between the index of refraction of the polymer 332 and that of the liquid crystal droplets 331 with the incident light consequently scattered.
On the other hand, when the voltage is applied to each pixel electrode 15 as shown in FIG. 34B, the liquid crystal molecules are aligned in one direction. If the index of refraction of the liquid droplets exhibited when the liquid crystal molecules are aligned in one direction is matched with that of the polymer 332, the incoming light passes through the array substrate 12 without being scattered.
The PD liquid crystal display panel of the type discussed above is manufactured in the following manner. As the polymer 332, light curable resin, particularly UV-curable resin, is generally employed. The array substrate 12 and the counter substrate 11 are retained in position spaced a predetermined distance from each other by a retaining means which is often employed in the form of fine beads. No orientation film 382 is basically needed in the PD liquid crystal display panel. A solution containing a mixture of UV-curable resin and liquid crystal material (hereinafter referred to as a LC mix) is injected into a space between the array substrate 12 and the counter substrate 11, followed by radiation of ultraviolet (UV) rays to cure the UV-curable resin. Upon curing of the UV-curable resin, the resin component and the liquid crystal component are phase separated. Where the quantity of the liquid crystal material is relatively small, it forms the liquid crystal droplets 331 as shown in FIGS. 34A and 34B, but where it is relatively great, the liquid crystal droplets 331 are continuously connected.
Portion of the UV-curable resin which has been radiated by ultraviolet rays of light is cured to result in phase separation between the resin component and the liquid crystal component, but the remaining portion of the UV-curable resin which has not been radiated remains uncured. An example in which, in the structure of the liquid crystal display panel shown in FIG. 60, the LC mix referred to above is injected in place of the TN liquid crystal material 383 will be discussed. Since the black matrix 381 is in the form of the metallic thin-film, it serves to shield the ultraviolet rays of light off. Since the thin-film transistor 14 and other components are also in the form of metallic thin-films, they also serve to shield the ultraviolet rays of light off. Accordingly, that portion of the UV-curable resin within an area underneath the black matrix 381 does not cure even though radiated by the ultraviolet rays of light from the side of the counter substrate 11. This is partly because, when the ultraviolet rays of light are radiated in a direction indicated by the arrow A, the black matrix 381 shields the incoming UV rays of light and partly because, when the ultraviolet rays of light are radiated in the opposite direction indicated by the arrow B, the thin-film transistors 14 shields the incoming UV rays of light.
The presence of that portion of the UV-curable resin which has been left uncured adversely affects the reliability and the lifetime of the PD liquid crystal display panel. Specifically, not only does the composition of liquid crystal material tend to vary while the liquid crystal display panel is operated, but also the liquid crystal layer and the counter substrate 11 are apt to be separated.
The presence of the black matrix 381 makes it difficult to fix UV radiating conditions during the manufacture of the display panel. At the time of manufacture, the LC mix in which the liquid crystal material and the UV-curable resin are mixed in a predetermined mixing ratio is injected into the space between the array substrate 12 and the counter substrate 11 and is subsequently radiated by the ultraviolet rays of light, the average particle size of the liquid crystal material around each pixel electrode 15 (adjacent to the black matrix) or the average pore size of a polymer network tends to increase. This is supposed to be because the black matrix 381 absorbs and is therefore heated by the ultraviolet rays of light, resulting in a localized increase in temperature of the resin around the black matrix and a localized change in condition by which the liquid crystal component and the resin component are phase separated. The scattering characteristic is also reduced. As hereinabove discussed, if the black matrix 381 is formed, even the slightest change in temperature during the manufacture and the intensity of UV radiation bring about a considerable change in average particle size of the liquid crystal material overlaying the pixel electrodes or in average pore size of the polymer network overlaying the pixel electrodes, thereby imposing limitations on the manufacturing condition. Accordingly, it is difficult to manufacture constantly the polymer dispersed liquid crystal display panels having an equal operating characteristic.
Where the polymer dispersed liquid crystal display panel is used as a light valve in the projection type display device, the presence of that portion of the UV-curable resin which has been left uncured in the display panel tends to constitute a cause of considerable deterioration in performance of the polymer dispersed liquid crystal display panel. This appears to result from the fact that, in the projection type display device, light of an intensity of about some ten thousand luxes or higher falls on the display panel, subjecting the latter to light-induced and heat-induced stresses.
As hereinbefore discussed, in the prior art TN liquid crystal display panel, the efficiency of utilization of light is relatively low because of the use of the polarizing plate, making it impossible to accomplish a high luminance display. Also, though the high luminance display can be attained with the polymer dispersed liquid crystal display panel, the prior art structure is instable because of that portion of the UV-curable resin remaining uncured and cannot be employed in practice. Also, the manufacturing tolerance thereof is extremely limited, making it difficult to obtain constantly the polymer dispersed liquid crystal display panel having a high light scattering characteristic.